Coherent Image Formation for Dynamic Transmit Beamformation

ABSTRACT

Retrospective dynamic transmit beamformation is provided in medical ultrasound imaging. Using parallel receive beamformation, sets of data representing locations in at least a common field of view are obtained, each set in response to a transmit with a spatially distinct phase front. The common field of view receive data are time aligned and amplitude weighted for retrospective transmit focusing and retrospective transmit apodization, respectively. A time offset, such as of a cycle or more in some cases, is applied to the receive data for retrospective transmit focusing. The offset is selected to emulate shifting the transmit delay profile to be tangentially intersecting with the dynamic receive delay profile for each location which is the desired transmit delay profile. A weight is applied to the receive data for retrospective transmit apodization. The weight is selected based on the desired transmit apodization profile. The offset and weighted data representing a same location from different transmit events is coherently combined. The number of sets of data offset, weighted and combined may vary as a function of depth for dynamic transmit beamformation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent document claims the benefit of the filing date under35 U.S.C. §119(e) of Provisional U.S. Patent Application Ser. No.61/059,668, filed Jun. 6, 2008, which is hereby incorporated byreference.

BACKGROUND

The present patent document relates to coherent combination of receivedultrasound signals. In particular, coherent combination of receive datais provided for retrospective dynamic transmit beamformation.

Conventionally to generate a two-dimensional image, acoustic energy istransmitted along a plurality of scan lines sequentially. The transmitbeam is focused at one location along the scan line. In response to eachtransmission, echo signals are received along the respective scan line.The receive beam is dynamically focused as a function of depth (time)along the scan line. Transmit focus is static, and receive focus isdynamic.

For more rapid acquisition, particularly for three-dimensional imaging,a broader transmit beam is formed. Echoes are dynamically received alonga plurality of scan lines in response to the broader transmit beam.However, beam group artifacts and loss of signal-to-noise-ratio (SNR)may result. For greater numbers of receive beams formed for eachtransmit beam, a greater beam group artifact and a greater loss of SNRmay result due to the static transmit focus.

Phase alignment and coherent processing of the receive data may reducethe beam group artifact. Coherent receive data representing the samelocations but from two spatially adjacent transmissions are phasealigned to account for the difference in transmit focus. The phasealignment is based on the position of the transmit focus. After phasealignment, the data representing the same locations is then coherentlycombined.

The number of acquisitions may be reduced by interpolating data alongscan lines. Received data representing different scan lines is phasealigned and combined to represent an intermediary scan line. However,artifacts due to the static transmit foci may still result.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, instructions, computer readable media, and systems forretrospective dynamic transmit beamformation in medical ultrasoundimaging. Using parallel receive beamformation, sets of data representinglocations in at least a common field of view are obtained, each set inresponse to a transmit with spatially distinct static phase front. Thedistinct static phase fronts are achieved through delay profilessteering and focusing at different locations. The common field of viewreceive data are time-aligned and amplitude-weighted for retrospectivetransmit focusing and retrospective transmit apodization, respectively.A time offset, such as of a cycle or more in some cases, is applied tothe receive data for retrospective transmit focusing. The offset isselected to emulate shifting the transmit delay profile to be tangentialwith the receive delay profile for each location. A weight is applied tothe receive data for retrospective transmit apodization. The weight isselected based on the desired transmit apodization profile. The offsetand weighted data representing a same location from different transmitevents is coherently combined. The number of sets of data offset,weighted and combined may vary as a function of depth for dynamictransmit beamformation.

In first aspect, a method is provided for retrospective dynamic transmitbeamformation in medical ultrasound imaging. First and second transmitbeams having first and second foci, respectively, are transmitted. Thefirst focus is different than the second focus. First and second sets ofmultiple receive beams are generated in response to the first and secondtransmit beams, respectively. Each of the receive beams has coherentsamples representing at least overlapping locations in a field of view.For each of a plurality of locations, an at least one cycle delay offsetis applied to at least one of the coherent samples. The offset is afunction of the first focus and a receive focus for the coherent sample.Coherent samples from the first and second sets are combined for eachlocation after applying the offsets. The coherent samples representingeach location are a function of the offset. An image representing thefield of view is generated. The image is a function of the combinedcoherent samples.

In a second aspect, a computer readable storage medium has storedtherein data representing instructions executable by a programmedprocessor for retrospective dynamic transmit beamformation in medicalultrasound imaging. The storage medium includes instructions foracquiring data from multiple pulse-echo acquisition events, eachpulse-echo acquisition event corresponding to different transmit delayprofiles, aligning the data with offsets, the offsets corresponding to atemporal shift in one of the transmit delay profiles to a tangentialintersection with receive wavefronts for the data, combining the aligneddata, and generating an image as a function of the combined data.

In a third aspect, a method is provided for retrospective dynamictransmit beamformation in medical ultrasound imaging. A plurality oftransmit beams having different foci are transmitted. Sets of multiplereceive beams are generated in response to the respective transmitbeams. Each of the receive beams of a set has coherent samplesrepresenting different locations in a field of view. At least onereceive sample of each receive beam set is common among the sets. Foreach location, an offset is applied to at least one of the coherentsamples. Coherent samples from the sets are combined for each locationafter applying the offsets. A number of sets contributing data to becombined varies as a function of depth. An image representing the fieldof view is generated as a function of the combined coherent samples.

In a fourth aspect, a computer readable storage medium has storedtherein data representing instructions executable by a programmedprocessor for retrospective dynamic transmit beamformation in medicalultrasound imaging. The storage medium includes instructions fortransmitting a plurality of transmit beams having different foci,generating sets of multiple receive beams in response to the respectivetransmit beams, each of the receive beams having coherent samplesrepresenting locations in at least a same region in a field of view, foreach location, retrospectively focusing the transmit beams to thelocation by altering the coherent samples, combining coherent samplesfrom the sets for each location after retrospectively focusing, andgenerating an image representing the field of view, the image being afunction of the combined coherent samples.

In a fifth aspect, an ultrasound image formation method is provided. Aplurality of receive beam sets are formed. Each set uses echoes receivedin response to a transmit event with spatially distinct phase front andeach set representing at least a common region as another set. Receivedata for a plurality of the sets is aligned, in depth, prior toamplitude detection. A synthesized line is generated throughpre-detection summation of depth aligned receive data.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments and may be later claimedindependently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a flow chart diagram of one embodiment of a method forretrospective transmit focusing in medical ultrasound imaging;

FIGS. 2 and 3 are example charts representing a data acquisition format;

FIGS. 4 and 5 are graphical representations of one embodiment ofdetermining an offset for receive data to retrospectively focus thetransmit; and

FIG. 6 is a graphical representation of one embodiment of a region ofaperture contribution of a given transmit wavefront to a combination ofcoherent samples;

FIG. 7 is a graphical representation of the contribution of differentsets of data as a function of depth in retrospective transmit focusing;and

FIG. 8 is a block diagram of one embodiment of a system forretrospective transmit focusing in medical ultrasound imaging.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In retrospective transmit focusing, signals from multiple acquisitionevents that are in-focus with respect to receive are added pre-detection(synthesized) to reconstruct the signal that would have been received ifacquired with a focused transmit at each. One way to achieveretrospective transmit focusing is to transmit with each element of thearray in turn while receiving along the same ultrasound line or linesfor each acquisition. Next, for each receive focus position and arrayelement, the appropriate focusing delay is applied prior to summing themany signals. However, the number of transmit events required to samplethe entire aperture is great, which may not be rapid enough, may haveinsufficient field strength for harmonic imaging, and may be susceptibleto motion artifact.

In much the same way that there are many basis functions that can beused to decompose a signal (e.g. delta function for standarddigitization, sinusoids for Fourier decomposition, etc.), there are manyways to acquire the lateral wave number content, or “look angles,” thatthe available transmit aperture may contribute to the synthesis oftransmit (and therefore round-trip) focus. This understanding, inconjunction with an acquisition scheme that progresses a group ofreceive focus beams along with a set of transmit aperture function“basis functions”, provides a practical implementation of retrospectivetransmit focusing on a system capable of receiving multiple (e.g.,several, tens or hundreds) receive beams in response to each transmitbeam. The transmit focus synthesized for progressive scanning using thetransmit array with spherical wavefront transmit delays (conventionalfocus, or virtual point source).

In one embodiment of retrospective transmit focusing, each samplerepresenting each location is formed from signals received from multipletransmit-receive pulse echo receive events. The several pulse echoevents have different transmit focusing delays. With large receive beamcounts (e.g., several, tens or hundreds) per transmit and scanning in aprogressive fashion, data from each transmit may be used in synthesizingseveral receive lines without having to re-do pulse echo receive events.

For synthesis, echo data is adjusted in time and possibly phase. For thesynthesis of the focused transmit aperture, time only may be sufficientif applied at the ultrasound frequency. If applied at baseband, phaseadjustment may be used in addition to time. Phase only correction may beinsufficient to achieve substantial transmit aperture synthesis, thoughphase only correction may provide synthesis in some situations. Thecorrection is a function of the transmit focusing delay, which is staticfor each pulse echo receive event, and the receive focusing delay, whichis dynamic during the pulse echo receive event. The time correction isapplied in the receive beamformer receive delay profile, providingaccurate receive focusing delay for the corrected range. Alternatively,the time correction is applied after beamforming before or afterdemodulation. The phase difference that results from this difference insignal processing functions may be accounted for by phase adjustment.

Due to adjusting in time and/or phase, the most substantial part of thesignals from the multiple events arrive in-phase with respect to echoesproduced by backscatter from the current point of interest. The transmitand/or receive idealized focused wave front at time zero may beconsidered as a circle whose center is at the focus and that intersectsthe line origin at the transducer/body interface. The appropriatecorrection to apply for retrospective transmit focusing is then foundconceptually by considering the current actual transmit used to acquireddata and the to-be-synthesized delay for the current point of interest(i.e., location in the field of view). The to-be synthesized delay isbuilt by addition of several different actual curves. To find theappropriate time correction, the actual curve is evolved until itintersects tangentially with the to-be-synthesized curve. This point ofintersection lies along the line between actual and to-be-synthesizedfocus points, which simplifies the calculation. The time correction toshift the actual transmit delay is applied to the received data. Theinformation that is contributed by the transmit wavefront is associatedwith the echoes whose actual wavefronts are aligned with the focusedwavefront to within some tolerance. Rather than selecting a sample forthe given location, a sample from a different location is selected basedon the time correction.

More transmit events may contribute meaningfully to the synthesizedoutput at shallow depths than at deep depths. The transmit events to besynthesized are controlled as a function of geometry. For example,range, and possibly line dependent scaling, is provided in the synthesisfunction. The scaling provides gradual transitioning of beams from “on”to “off.” A transmit event ceases to be good for synthesis at deeperdepths, because the source of those echoes as given by tangentialintersection of actual and desired wavefronts is outside the extent ofthe array or aperture.

Any transmit delay profiles may be used. In the discussion below forFIG. 1, conventionally focused and spherical diverging transmit delayprofiles are used. For small aperture probes having scan lines of commonorigin, circular delay profiles make for regularly spaced “look angles.”However, other transmit delay profiles may be used. It is a matter ofthe mathematics to determine what time adjustment provides tangentialintersection with the to-be-focused delay profile in order to make useof other transmit profiles.

Focused transmits are used in the example embodiment. Focused transmitskeep the field strength and, therefore, harmonic levels reasonably highas compared to substantial defocusing. The foci may be positioned in oroutside of the field of view. For example, the focal depth is greaterthan the field of view (e.g., deeper than the image depth). In otherembodiments, defocused, plane waves or other transmit delay profiles areused.

Any scan format may be used, such as sector or vector format on smallaperture probes, or linear format on large aperture probes. The conceptis not specific to those cases. In small aperture probes, conventionalfocused and diverging spherical transmit delay profiles yield lookdirections for the contributed information that are regularly spaced andrange-invariant for the sector format. Large aperture probes may beused. The probes may be one-dimensional or multi-dimensional arrays ofelements.

Deficiencies in beamformer beam count may be compensated for by multiplepulse echo events. Multiple pulse echo events prior to advancing thescan in azimuth may be used to synthesize the transmit focus in theelevation dimension. If a scan is small enough in azimuth, it may bepossible to use this method in both azimuth and elevation dimensionswithout incurring too much motion artifact. This method may beadvantageous if compensating for speed of sound in the body that isdifferent from the usual assumption (manually, adaptively, orotherwise), because a large part of this correction may be taken care ofwithout actually changing transmit characteristics and therefore notinvolving acoustic output considerations.

Another way of expressing the retrospective transmit focusing iscombining data from multiple pulse-echo acquisition events where thetransmit delay profiles differ and the receive delay profiles aresubstantially the same except for a channel independent offset thatcreates substantially in-phase summation across the various transmitevent data when considering a coherent target. Other features may beprovided. The different transmit delay profiles may provide independentregions over the array where their slope is similar to the receive delayslope for each image location. The different transmit delays may providesubstantially different focus laterally. The delay offset may be greaterthan a half-wavelength. A large offset may be used for synthesizing atransmit focus retrospectively where a smaller offset that may beappropriate for synthetic line interpolation may not work. The delayoffset may be computed as the time evolution required to achievetangential intersection between the actual transmit time zero wavefrontand an actual propagation wavefront emanating from the point of interestand intersecting the line origin. The lateral spectral bandwidth may bemade substantially more linear-phase due to the synthesis. In thefrequency domain, the phase of the spectrum can be made linear over alarge band of lateral frequencies. In k-space, retrospective transmitfocusing provides more linear lateral spectral bandwidth than withoutthe retrospective transmit. The synthesis count may be modulated asfunction of range. Lateral interpolation (analytic beam interpolation)may be provided with the synthesis operation, increasing the frame rate.The interpolation may be provided prior to synthesis for retrospectivetransmit focus.

FIG. 1 shows a flow chart for a method for retrospective transmitfocusing in medical ultrasound imaging. The method is implemented by oron the system of FIG. 8 or a different system. Additional, different orfewer acts may be provided. For example in an embodiment, the methoddoes not include act 22. As another example, the method does not includeact 30 in an embodiment. The acts are performed in the order shown or adifferent order, according to various embodiments.

In act 12, data is acquired from multiple pulse-echo acquisition events.Each pulse-echo acquisition event corresponds to different transmitdelay profiles and reception of a plurality of receive beams in responseto each transmit beam. Two or more pulse-echo acquisition events areperformed for forming a given image. The receive data generated from thepulse-echo acquisition events is processed by applying an offset andcombining for retrospective transmit focusing. The acquisition isperformed by transmission of ultrasound in act 14 and reception ofresponsive echoes in act 16.

In act 14, a plurality of statically focused transmit beams aretransmitted. In one embodiment, the transmit beams are sequential.Sequential transmit beams allow for the reception of act 16 to occurwith the same transducer array between transmitting each transmit beam.In other embodiments, the transmit beams are coded differently andtransmitted simultaneously. The coding allows reception of data specificto the given transmit beam. Sequential transmit beams will be used belowas an example.

At least two, and possibly all, of the transmit beams have differentfoci. The transmitted beam converges at the focal location. Inalternative embodiments, plane waves or defocused waves (e.g., divergingwavefront) are used where the difference in foci correspond to differentwave origins and/or directions. The different foci are provided by usingdifferent transmit delay profiles and corresponding channel delaysand/or phasing. The transmit waveforms have different wavefronts due tothe different foci. The same transmit aperture is used, but differenttransmit apertures may be provided for different transmit beams. Forconverging wavefronts, the different foci are at different locations,but may alternatively or additionally have differences in size and shape(e.g., line focus). The different foci are at different laterallocations. For example, the different foci are spaced in azimuth but ata same depth. Different depths may be used instead of different laterallocations or in addition to different lateral locations. The foci arewithin the field of view or imaging region. Alternatively, the foci areoutside the field of view. For example, all of the foci are deeper thanthe field of view. The field of view of the image is between atransducer used for the transmitting and the foci. Rather than having aninfinite focus (e.g., plane wave), the foci are within a patient or aregion near the patient. For example, the foci are laterally spaced inazimuth and/or elevation within four times (e.g., at four times, atthree times, at twice, or at 1.5 times) a depth of the field of view. Inone example, the depth of the field of view is 18 cm and the focal depthis 45 cm.

The transmit beams propagate along a particular nominal transmit beamaxis or transmit line to the respective focal location and beyond. Eachtransmit beam has an origin in common with other transmit beams, but mayhave a different origin (e.g., azimuthally spaced origin using a lineararray). The transmit beams are noncollinear. The point spread functionof the transmit beams may avoid substantial overlap at a given intensity(e.g., 6, 10, or 20 dB down from a maximum). Avoiding “substantial”overlap accounts for overlap used to receive along adjacent scan linesduring a scan. The transmit beams may differ or be the same in one ormore of the transmit beamforming and pulse shaping parameters, such asfocal depth, center frequency, apodization type, aperture width,bandwidth or other transmit beam characteristic in addition to havingdistinct transmit lines.

In act 16, sets of multiple receive beams are generated in response toeach respective sequential transmit beam. The receive beams are formedby dynamic focusing with receive beamformers. For each receive beam,different delay profiles are applied as a function of time or depth. Thedynamic focusing results in coherent samples representing differentlocations in a field of view. Samples are provided along each of thereceive beams.

Any number of receive beams and corresponding samples may be generatedfor a given transmit beam. For example, eight, sixteen or other numberof receive beams are generated for a given transmit beam. The samplesfor the receive beams represent the ultrasound response alongcorresponding scan lines. The samples from a given transmit event are aset of data for that transmit. The scan lines are spaced apart in a scanregion. The scan region is the entire field of view or only a portion ofthe field of view. For each given location in the field of view, aplurality of samples is provided. The receive beams from differenttransmissions may overlap or intersect. Multiple sets of receive dataare available for each location, such as two, three, eight, sixteen orother numbers of sets being acquired for a given location.

In act 12, a scan sequence of acts 14 and 16 is performed to scan thefield of view. For example, the scan sequence uses three transmit beamsfor a three-way synthesis. In one embodiment, three-way synthesis isprovided at the maximum expected display depth where the componenttransmit focal length is close to the synthesized focal length so thatthe bandwidth contributed by each component is substantial. Forming animage over typical display depth ranges may use a greater number ofreceive beams for a given location so that the available transmitaperture may be fully represented in the synthesized output at eachdepth. In the near field, the bandwidth contribution of each componentis small, and many acquisitions from many angles may be used. Forexample, eight, sixteen, or more samples are provided for each nearfield location for greater representation of the transmit aperture thanprovided using fewer samples. The maximum number of beams per group thatcontribute to synthesis may be limited by the simultaneous receive beamcapability of the beamformer, but might also be limited by motion of thetarget object or a desired transmit F-number design choice.

One embodiment of a progressive scanning configuration for three-waysynthesis is represented in FIG. 2. The parameters listed are arepresentation of the configuration. The capital letters “T” indicatecomponent transmit nominal beam steering, the integers on each separaterow indicate nominal lateral beam steering of each receive beamassociated with a single acquisition event, relative to a first beam ofthe last event, the letters “u,” “s,” and “d” indicate the nominal beamsteering of resulting synthesized ultrasounds beams of the ramp-up edge,steady-state scanning, and ramp-down edge, respectively. In thisschematic representation, enough groups to show one steady-stateacquisition are shown.

Other parameters correspond with pre-synthesis interpolation (e.g.,Analytic Beam Interpolation, or ABI) alignment, represented as abiAlign,the extrapolation of beam group by conceptually padding receive beamgroup data with zero-value beams on each edge (abiExtrap), the receivebeams acquired per group (numGroup), the number of post-abi beams whichtranslate the beam group per acquisition (gslSlip), the numbers oframp-up and ramp-down lines to be passed through the line synthesisoperation (outsIncmp), and a number of acquisitions where the receivebeam group does not translate (gslSum). Pre-synthesis interpolation maynot be provided in other embodiments.

Scanning in the progressive fashion represented in FIG. 2 allows foreffective use of a limited number of simultaneously acquired beampositions because the receive beams most aligned with transmit areacquired around each group and with as many additional receive beams ascan be supported by the system, desired transmit F-number, or motionartifact constraints. The configuration depicted may not be an optimalone in terms of lateral sampling efficiency. FIG. 3 shows anotherexample scan sequence with a pre-synthesis up-sampling design. The “+”symbols indicate the nominal beam steering positions of datainterpolated from multiple receive beams of an acquisition. Theinterpolation is performed prior to synthesis so that interpolatedreceive beams are used in the synthesis. The scheme of FIG. 3 allows forthe sampling density of the originally acquired multiple receive beams(indicated by integers in the above schematic) to be designed accordingto the lateral bandwidth of component images and subsequently up-sampledto support an increase in lateral bandwidth that is attendant to theretrospective transmit focus synthesis.

The interpolation scheme represented in FIG. 3 does not use the originalreceive beams, but instead relies only on the interpolated beams. Thesynthesized outputs, in terms of both the relative distances tobeamformer beams and the number and scaling weights of those beamcontributors to each interpolated output, may be more uniform. Theoriginal receive beams may be used in addition to the interpolated beamsin other embodiments.

Other variations to the scan configuration may include alternate choicesfor translation of the beam group per acquisition and using thecapability to sum acquisition events of different transmitcharacteristics without progressing the receive group. Varying thetranslation of the beam group per acquisition allows for the acquisitionrate to be substantially modified in exchange for altering thecharacteristics of the round trip synthesized aperture function, andtherefore the lateral point spread function for each beam, as well asthe uniformity of this point spread function from across synthesizedoutputs. The capability to sum acquisitions without translating beamgroup allows for such operations as non-progressing transmit focussynthesis, which may be useful for synthesis in the plane perpendicularto the scanning direction. This method is applicable in volume scanningas well as planar. Due to target motion, progressive synthesis only inthe direction of lateral scanning may be used and non-progressivesynthesis may be used in the non-orthogonal direction. Alternatively,multiple acquisitions are used to compensate for an insufficiency ofsimultaneous receive multi-beam capability in the beamformer. Phaseinversion or harmonic pulse sequence weighting using phase differencesin the transmitted waves may be provided before or after synthesis fortransmit focusing.

The receive beam density is set to sufficiently sample the informationcontent of the individual acquisition components. For example, for smallaperture transducers where the receive beams are spaced uniformly inangle or sine of angle, the beam spacing may be set to λ/(2 W) inradian, where λ is the wavelength and W is the aperture size, both inunits of mm. For transmit designs that do not contribute much lateralbandwidth prior to synthesis, this is a one-way focused imagingcriterion. Indeed, the single component lateral bandwidth may be muchless than the round-trip synthesized bandwidth, because in thissituation the acoustic data is sampled according to the per-componentbandwidth. Analytic beam interpolation (ABI) is used to prevent aliasingthrough the synthesis of the additional bandwidth due to the transmitfocus and realize up to a factor of two acquisition rate advantage overconventional round-trip focusing.

With the receive beam spacing determined, the minimum transmit F-numberthat can be synthesized may be approximately determined by assuming thateach acquisition contributes a narrow band of lateral wave numbersaligned with the look directions of the receive beams. The minimumsynthesized F-number may be close to the inverse of two times thetangent of one less than the receive beam count times the receive beamspacing.

In act 18, the sequential transmit beams are retrospectively focused.The focusing is performed differently for each location in the field ofview. Data from multiple sets of coherent samples are altered forretrospectively transmit focusing. Each set is associated with adifferent transmit beam and includes data from a plurality of locations.By altering the coherent samples associated with a given location, thetransmit focus may be adjusted. The data for a given location isselected from data representing a different location.

In act 20, an offset is applied to the coherent samples for eachlocation. The offset aligns the coherent data. For a given set, thesamples are shifted in range to represent different locations based onthe offset. For example, the locations may be shifted by a wavelengthfor a one cycle delay offset. The offset applied may be different foreach location. One or more locations may be associated with no offset,such as at a transmit focal location. Alternatively, an offset isdetermined for each of the sets of data.

In the embodiment discussed above, the offset is applied to beamformeddata. The offsets are delays. The delays may be one or more cycles. Forsome locations, less than a cycle delay may be used. A phase adjustmentmay be used for offsets of less than a cycle or for non-integer offsets(e.g., an integer cycle delay plus a phase offset). In otherembodiments, the offsets are applied to channel data. Delays and/orphase adjustments are applied as offsets to the signals from eachelement prior to beamformation. The offsets may or may not be a singlecycle or longer.

For a given location, the offset is a function of the transmit focus andthe receive focus. The coherent sample in a given set of data associatedwith a transmit focus is selected using the transmit focus and receivefocus relative to the location for which data is determined. By applyingthe offset to the samples, the data for retrospectively focusing isselected for the location. The offsets for the data from different setsfor a given location correspond to temporal shifts in one of thetransmit delay profiles to a tangential intersection with the focuseddelay profile for the data. Each data set contributes a portion of theretrospective focusing.

To understand what each component acquisition contributes to thesynthesized signal, consider the Fourier transform model for focusedapertures. Within the focal plane, the lateral point spread function isgiven by the Fourier transform of the apodization function. Within thisformalism, the effect of defocusing in range is modeled by introducing aphase variation onto the apodization function that arises from thedifference between the physical path-length to the target and that,which is assumed by the focusing delay calculation. This results in anintegral suited to the method of stationary phase integration. Thephysical basis of which is the recognition that the main contributor tothe integral result is the portion of the kernel over which the phase isstationary or non-varying. Extending this idea to the componenttransmits allows determination of the “look angle” for the lateral wavenumbers or region of support of the apodization function in the Fouriertransform model that are contributed by each acquisition, as well asestimate of the bandwidth contributed by each component. The look angleand magnitude of shift determine the advance or offset to apply prior tosynthesis.

FIG. 4 shows the relationship between transmit and receive delayprofiles for a given location, indicated as the receive (rx) focus dueto dynamic receive operation. The array surface, a receive focus pointand corresponding spherical wavefront corresponding to time zero, and atransmit (tx) focus point for the set of data (i.e., one componentimage) and corresponding spherical wavefront corresponding to time zeroare shown. For spherical wavefronts, the line intersecting the twocenters, or foci, also intersects the spherical wavefronts at the pointswhere tangential or parallel intersection of spherical wavefronts forthose foci occurs. This line is marked as the angle to tangency.

FIG. 5 shows the transmit wavefront evolved in time to achievetangential intersection with the wavefront corresponding to the receivefocus at time zero. This receive wavefront is also the proper referencefor the focused transmit wavefront that should be synthesized. Thetemporal difference or shift is the offset. The angle to tangency lineindicates the direction of the shift. The application of a time- orrange-adjustment to the acquired component data effects the evolution intime.

The contribution of this component data is dominated by the lateral wavenumbers corresponding to the tangential intersection shown. Practicalacquisition sequences provide for multiple acquisition components thatallow the full extent of the physical aperture to be reconstructed within-phase synthesis components for most of the region of practical imageacquisition. Using laterally spaced transmit foci, the region of theaperture associated with the intersection shifts along the aperture.Each set of data contributes components for different portions of theaperture (e.g., the location of intersection and immediately surroundingportion of the delay profile is at different locations along thetransmit wavefront or aperture).

The offset determination is repeated for all receive foci and availableacquisition components. By applying offsets to respective sets for eachlocation, the different transmit delay profiles correspond to differentregions of the transmit aperture. At the tangential intersection, thetransmit delay profile has a slope similar to the slope of the receivedelay profile. For example, by setting a criteria of ¼ wavelengthdisparity between the tangentially intersecting spherical wavefronts, anestimate of the region of support, or lateral bandwidth, of the aperturethat is contributed by this component acquisition is set. FIG. 5 showsthe in-phase contribution region of the aperture provided by one set ofdata. Larger or smaller wavelength criteria may be used.

The acquisition components are adjusted in terms of the time, or groupdelay, for the purpose of synthesis. By using a true time delay, anadditional phase adjustment may not be needed to satisfy coherentsynthesis. Where the coherent samples are shifted after demodulation, aphase adjustment may be used in act 22 to account for the demodulation.Phase adjustment may also be applied for other purposes, such as lateralinterpolation.

In act 24 of FIG. 1, the retrospective focusing is completed bycombining the aligned data. Coherent samples from two or more sets arecombined for each location. The combination occurs after applying theoffsets for retrospectively focusing. The samples representing a givenlocation are each a function of a respective offset. The combination isa synthesis of coherent data prior to detection. In-phase andquadrature, radio frequency, or other data including phase informationis combined. The resulting coherent sample is formed from signalsreceived from multiple transmit-receive pulse echo receive events.

For each location, the different transmit delay profiles for the databeing combined contribute to different regions of an aperture. Theregion of the aperture is a function of the transmit focal location. Theregion of the transmit aperture has a slope similar to a receive delayslope for the location. A lateral spectral bandwidth of the combinedcoherent samples is more linear than for the coherent samples beforecombining. The phase of the lateral spectrum of the combined coherentsamples is more linear in frequency than for coherent samples beforecombining.

The combining is repeated for each location. Since the offsets are afunction of the location, the samples being combined for one locationmay be associated with different offsets than the samples being combinedfor another location.

For a given location, fewer than all of the sets or associated transmitfocal locations may contribute to the synthesis. For example, receivebeams may not have been acquired for a given location. As anotherexample, the region of lateral frequency support provided by a giventransmit beam may correspond to an echo source location that is outsideof the physical extent of the transducer array. In act 26, a number ofthe sets contributing data to be combined varies as a function of depth.The number is typically greater at shallow image depth and lesser atdeeper image depth.

As the receive focus moves deeper, the region of support contributed fora given focus that is offset in beam steering from the receive focusmoves outward. Eventually the region of support moves beyond the extentof the physical array or aperture. Such an acquisition may only orprimarily contribute clutter and is not included in the synthesisoperation.

The in-phase contribution of each candidate acquisition is considered inrelation to the physical constraints of the transmit aperture andapodization used to acquire the data. Acquisitions not expected tocontribute physically meaningful information are not used for a givendepth. The number of sets contributing to a synthesis is modulated as afunction of depth. In addition, by varying the synthesis weights, it isalso possible to shape the round trip aperture function that resultsfrom synthesizing, effectively apodizing retrospectively. Such shapingof the aperture function may mitigate lateral sidelobes or may becontrolled to whiten or produce a more flat-topped, round-trip aperturefunction with greater detail resolution.

As the receive focus moves deeper, the width of the region of supportcontributed by the remaining acquisition components increases. At thelimit where the transmit and receive focal lengths are equal, the singleacquisition component that aligns in beam steering with the receivefocus contributes all available lateral wave numbers and no othercomponents are needed. This equates to conventional confocal imaging.The full lateral bandwidth of round trip focusing is present in thesingle acquisition component, and therefore must be sampled laterallywith greater density than the individual components acquired when thetwo focal lengths are disparate. This may be avoided by keeping thecomponent transmit focus well outside of the imaging field of view. Inone embodiment, the near field uses eight, sixteen, or other number ofcontributors or sets, and the far field uses two, three, or other numberof contributors.

The simple geometrical arguments above apply equally well when thecenter of the transmit component wavefront, or focus, is behind thephysical array. This situation is called a virtual point sourcetransmit. The concept is not restricted to spherical wavefront transmit,though this case provides significant simplifications and seems wellsuited to the method. The principle of this method is not restricted tosector format.

The combination of the offset data provides a synthesized aperture. Thetransmit design, which may be spherical or cylindrical (diverging,converging, or plane) wavefronts producing an imaging depth of fieldoutside the field of view, is modeled with the scanning sequence, whichmay be a progressing group of multiple dynamically received beams whosebeam steering surrounds the beam steering of transmit. FIG. 7 shows thesynthesized transmit aperture function as it evolves with image depth.In the near field, a relatively constant F-number is achieved with afixed synthesis count. As depth increases, the need for synthesis countto drop is illustrated as the remaining bandwidth of the remainingsynthesis contributors increases to still cover lateral spectral contentthat is available due to the physical aperture.

In act 28, information is detected from the combined coherent samples.Any detection may be used. For example, the intensity or amplitude ofresponse is determined with or without log compression. As anotherexample, flow or motion may be estimated, such as with Dopplerprocessing.

Prior to or after detection, other processes may occur. For example,other receive data may be interpolated from the synthesized coherentsamples. As another example, filtering is applied.

In act 30, the detected data is low pass filtered and decimated. The lowpass filtering and decimation may be used to match a downstream datarate or dataset size or display grid density limits. In otherembodiments, act 30 is not provided, interpolation, or other processesare used.

In act 32, an image is generated using the combined data orretrospective focusing. The detected data, after any processing (e.g.,filtering and decimation), is mapped to a display. Color and/or grayscale mapping is used. The detected information is mapped to displayvalues. The image represents the field of view. The field of view is atissue, bone, and/or fluid region in the patient.

FIG. 8 shows one embodiment of a system for retrospective focusing. Thesystem is an ultrasound imaging system, but other imaging systems usingan array of elements may be used. The system includes a transducer 33, atransmit beamformer 31, a receive beamformer 34, a coherent imageformer36, a detector 38, an incoherent imageformer 40, and a control processorand memory 42. Additional, different or fewer components may beprovided, such as the system with a CINE memory, scan converter and/ordisplay.

The transducer 33 is an array of a plurality of elements. The elementsare piezoelectric or capacitive membrane elements. The array isconfigured as a one-dimensional array, a two-dimensional array, a 1.5Darray, a 1.25D array, a 1.75D array, an annular array, amultidimensional array, combinations thereof or any other now known orlater developed array. The transducer elements transduce betweenacoustic and electric energies. The transducer 33 connects with thetransmit beamformer 31 and the receive beamformer 34 through atransmit/receive switch, but separate connections may be used in otherembodiments.

Two different beamformers are shown, a transmit beamformer 31 and thereceive beamformer 34. While shown separately, the transmit and receivebeamformers 31, 34 may be provided with some or all components incommon. Both beamformers connect with the transducer 33. The transmitbeamformer 31 is a processor, delay, filter, waveform generator, memory,phase rotator, digital-to-analog converter, amplifier, combinationsthereof or any other now known or later developed transmit beamformercomponents. The transmit beamformer is configured as a plurality ofchannels for generating electrical signals of a transmit waveform foreach element of a transmit aperture on the transducer 33. The waveformshave relative delay or phasing and amplitude for focusing the acousticenergy. The transmit beamformer 31 includes a controller for altering anaperture (e.g. the number of active elements), an apodization profileacross the plurality of channels, a delay profile across the pluralityof channels, a phase profile across the plurality of channels andcombinations thereof. A scan line focus is generated based on thesebeamforming parameters.

The receive beamformer 34 is a preamplifier, filter, phase rotator,delay, summer, base band filter, processor, buffers, memory,combinations thereof or other now known or later developed receivebeamformer components. The receive beamformer 34 is configured into aplurality of channels for receiving electrical signals representingechoes or acoustic energy impinging on the transducer 33. Beamformingparameters including a receive aperture (e.g., the number of elementsand which elements are used for receive processing), the apodizationprofile, a delay profile, a phase profile and combinations thereof areapplied to the receive signals for receive beamforming. For example,relative delays and amplitudes or apodization focus the acoustic energyalong one or more scan lines. A control processor controls the variousbeamforming parameters for receive beam formation.

Receive beamformer delayed or phase rotated base band data for eachchannel is provided to a buffer. The buffer is a memory, such as afirst-in-first-out memory or a corner turning memory. The memory issufficient to store digital samples of the receive beamformer 34 acrossall or a portion of the scan lines from a given range. The beamformerparameters used by the transmit beamformer 31, the receive beamformer34, or both are set for retrospective focusing. The beamformerparameters may be used as synthesis parameters for forming the componentbeams.

The receive beamformer 34 includes one or more digital or analog summersoperable to combine data from different channels of the receive apertureto form a plurality of receive beams. Cascaded summers or a singlesummer may be used. In one embodiment, the beamform summer is operableto sum in-phase and quadrature channel data in a complex manner suchthat phase information is maintained for the formed beam. The receivebeamformer 34 includes a parallel beamformer structure for forming aplurality of receive beams from the same received signals, such asforming tens or hundreds of receive beams with different delay profiles.

The transmit beamformer 31, transducer 33, and receive beamformer 34form a pulse-echo acquisition subsystem. The acquisition subsystemgathers three types of data, namely spatial domain, temporal domain, andparameter domain data. The spatial domain data provides structuralinformation in up to three spatial dimensions. The temporal domain dataprovides tissue motion and blood flow information. The parameter domaindata provides information on the angle/frequency dependence of tissueresponse, or on the acoustic properties such as nonlinearity, stiffness,or other parameters.

To acquire data, the transmit beamformer 31 of the front-end subsystemtransmits specially shaped and timed pulses into the body thousands oftimes per second. The receive beamformer 34 then generates multiplebeams through parallel and real-time processing (e.g., interpolation ofsynthesized beams) of echoes received in response to each transmittedpulse. The number of receive beams the receive beamformer 34 generatesin parallel determines the maximum information rate the imaging systemcan achieve.

The coherent imageformer processor 36 is a general processor, digitalsignal processor, control processor, application specific integratedcircuit, digital circuit, digital signal processor, analog circuit,combinations thereof or other now known or later developed processorsfor performing line synthesis. In one embodiment, the coherentimageformer 36 is part of the receive beamformer 34 or control processor36, but a separate or dedicated processor or circuit may be used inother embodiments. The coherent imageformer 36 includes memory buffers,complex multipliers and complex summers, but other components may beused.

The coherent imageformer 36 is operable to synthesize lines. Forexample, the coherent imageformer 36 is operable to form datarepresenting a range of depths or lateral locations from differentreceive beams or combine data from different sub apertures to form oneor more lines of data. Ultrasound lines are formed from receive beamsformed by the receive beamformer 34. The synthesis may involveinter-beam phase correction as a first step. Multiple stages or parallelprocessing may be used to increase the throughput or number of receivebeams processed for real-time imaging, such as associated with three- orfour-dimensional imaging. The synthesis then combines the phasecorrected beams through a coherent (i.e., phase sensitive) filter toform synthesized ultrasound lines.

In one embodiment, the coherent imageformer 36 includes pre-detectionparameter synthesis, axial filtering for receive pulse shaping anddecoding, phase correction to phase align receive beams in one or bothof the lateral axes, beam- and range-dependent gain for spatialweighting and/or masking of beams (i.e., weighting receive beams outsidea transmit beam region with a zero, such as for plane wave transmissionswith a sector or Vector® receive format) and line synthesis. Parametersynthesis combines receive beams responsive to transmit beams of likefocus. For example, the line synthesis is for phase inversion (receivebeams associated with transmissions with different, such as opposite,phases), contrast pulse sequences (receive beams associated withtransmissions at different amplitudes and/or phases), color flow, orother image forming processes coherently combining receive beams fromdistinct transmissions along a same scan line. Receive beams arecombined for line synthesis after any phase correction. The combinationis prior to detection or coherent. Any combination function may be used,such as summation, weighted summation or nonlinear combination ofreceive beams. The line synthesis is of receive beams responsive totransmit beams along different scan lines. As another example, the linesynthesis is for combination of receive beams formed in response todistinct transmit events or formed from data for a same transmit event.

Additional, different or fewer components and associated functions maybe provided by the coherent image former 36. Analytic beam interpolationforms new lines of data between receive beams from the sametransmissions, or new lines of data between receive beams of differenttransmissions but with like characteristics, such as interpolation amongdata of similar transmit to receive beam steering offset. Analytic beaminterpolation may increase the lateral sampling rate to prevent aliasingdue to noncollinear event synthesis. Pre-detection lateral filteringprovides lateral whitening or artifact reduction. Analytic lineinterpolation forms new lines of data between synthesized lines.Analytic line interpolation may increase the lateral sampling rate toprevent aliasing due to envelope detection.

The detector 38 is a general processor, digital signal processor,control processor, application specific integrated circuit, digitalcircuit, digital signal processor, analog circuit, combinations thereofor other now known or later developed processors for envelope detection.

The imageformer filters, synthesizes and compounds the multi-domain,multi-dimensional data acquired by the front-end into high qualityimages of one or more parameters of interest.

The amplitude detection and log compression stage separates theImageformer into two important subsystems, namely the coherentimageformer 36, which is for the phase sensitive processing of analyticsignals, and the non-coherent imageformer 40, which is for the phaseinsensitive processing of log-compressed amplitude (video) signals.

The coherent imageformer aligns the phase of the beams generated by thereceive beamformer and applies coherent or phase-sensitive processingacross phase-aligned beams. The coherent imageformer provides highquality multibeam operation through retrospective transmit focusing. Thecoherent imageformer captures the inter-beam phase information beforebeing discarded by the amplitude detection stage of the imageformer.This allows the imaging system to preserve information and sustain highinformation rate while improving the image quality of multibeamoperation.

The synthesis of lateral resolution after the beamformer is performedwith a lateral interpolation stage and also reduces the transmit countto increase frame rate. This interpolation stage may be performed priorto the synthesis so that aliasing that would result from the greatersynthesized bandwidth is avoided. Another lateral interpolation stagemay be prior to the detector 38 in each dimension to allow for thebandwidth widening detection processes without aliasing.

In one embodiment, the coherent image former 36 performs range and phaseadjustment to beam interpolation. The adjustments are performed as knownfor synthesizing additional receive beams. The analytic beaminterpolation is performed. A further range and phase adjustment isprovided for applying the offset for retrospective transmit focusing.The offset coherent samples are scaled and accumulated (synthesized) forthe field of view.

In another embodiment, samples for multiple receive beams are stored ina buffer. The receive beams are actually received beams and/orinterpolated beams. The offset is applied, and scaling and accumulationare performed. The samples for different locations are built upcontributor by contributor. As receive beams for a single transmit arereceived, the receive beams are adjusted by the offsets per location,scaled, and combined with any previous samples representing thelocation.

Where the range adjustment (e.g., offset for transmit focusing) providesthe capability to advance or retard the input signal with fractionalprecision and as a function of range independently for each data vectoror ultrasound beam, the adjusted samples contribute to the accumulation.Phase adjust, likewise, is a range and vector dependent phase rotationapplied to each synthesis constituent. The scaling of the input andpossibly the output of the synthesis accumulation is also range andvector dependent.

In one embodiment, the offset is performed with a processor, such as afield programmable gate array. Another processor implements any analyticbeam interpolation. The separate processor then applies the offset andaccumulation.

The incoherent imageformer 40 is operable on detected data to combineincoherently multiple ultrasound lines. In one embodiment, the input tothe incoherent imageformer 40 is the intensity data, and, in another,the input is log-compressed data. The ultrasound lines combined may havediffering temporal or spatial spectra. Sequential focus stitching (e.g.,zone cross-fade) may be performed in addition to frequency and spatialcompounding. Any extra transmit events that are not synthesizedcoherently may be combined incoherently or compounded to reduce speckleand improve image uniformity.

In one embodiment, the incoherent imageformer 40 includes buffers,filters, summers, multipliers, processors or other components forimplementing the compounding and/or other incoherent processes. Forexample, the incoherent imageformer 40 performs post-detection (video)axial filtering for receive pulse shaping, collinear multibeam spatialand/or frequency compounding, collinear transmit event compounding ofcorresponding collinear receive beams for transmit/receive frequencycompounding, sequential focus, transmit focus compounding, or otherpurposes, noncollinear transmit event compounding of collinear receivebeams for transmit/receive spatial compounding, post-detection lateralvideo filtering for lateral response shaping or artifact reduction, andadaptive gain, compression and mapping. Different, fewer or additionalincoherent processes may be provided.

In one embodiment, each coherent image former 36 and each incoherentimageformer 40 are operable for a limited number of channels, such as agroup of 16 channels. A plurality of devices is provided for each groupof channels. The outputs may then be used to synthesize further data orprovide further incoherent combinations. In one embodiment, theincoherent imageformer 40 is provided with a feedback from the detector38 for compounding detected data.

The images or receive beams combined coherently or incoherently are on asame acoustic or scan grid. Alternatively, a spatial transformation orscan conversion aligns the component beams or associated images. Thedata is output as an one-, two-, or three-dimensional representation onthe display. Other processes, such as the generation of text or graphicsmay also be performed for generating an image on a display. For example,a display dynamic range is set, filtering in space and time using alinear or nonlinear filter, which may be an FIR or IIR filter ortable-based, is provided, and/or the signal amplitude is mapped todisplay values as a function of a linear or non-linear map. Thenon-linear map may use any of various inputs, such as both filtered andunfiltered versions of the data being input in selecting a correspondingbrightness. Data optimized for contrast may be input with the same orsimilar data optimized for spatial resolution. The input data is thenused to select brightness or display intensity.

As part of the image forming process, the control processor 42 sets ascan pattern or acquisition sequence, number of simultaneous receivebeams, a number of sequential beams, a number of sub apertures, a numberof focal zones in a same scan line, a number of component beamscompounded together, receive multiple beam parameters, combinationfunction, component beam temporal frequency response, component beamspatial frequency response, a number of sets contributing toretrospective transmit focusing by depth, scan line density, focallocations, combinations thereof or other now known or later developedparameters for coherent combination by the coherent imageformer 36. Theparameters are set as a function of received ultrasound data and/or userinput. The received ultrasound data is from any where along theprocessing path, such as from the receive beamformer 34, the coherentimageformer 36, the detector 38 or the incoherent detector 40.

The instructions for implementing the adaptive processes, methods and/ortechniques discussed above are provided on computer-readable storagemedia or memories, such as a cache, buffer, RAM, removable media, harddrive or other computer readable storage media. The instructions areimplemented on a single device, such as the control processor 42, or aplurality of devices in a distributed manner. Computer readable storagemedia include various types of volatile and nonvolatile storage media.The functions, acts or tasks illustrated in the figures or describedherein are executed in response to one or more sets of instructionsstored in or on computer readable storage media. The functions, acts ortasks are independent of the particular type of instructions set,storage media, processor or processing strategy and may be performed bysoftware, hardware, integrated circuits, firmware, micro code and thelike, operating alone or in combination. Likewise, processing strategiesmay include multiprocessing, multitasking, parallel processing and thelike. In one embodiment, the instructions are stored on a removablemedia device for reading by local or remote systems. In otherembodiments, the instructions are stored in a remote location fortransfer through a computer network or over telephone lines. In yetother embodiments, the instructions are stored within a given computer,CPU, GPU or system.

An example embodiment of the instructions for retrospective or dynamictransmit focusing is provided. The number of transmit beams may belimited by motion and the number of simultaneously received receivebeams. The number of receive beams per transmit beam may depend on thehardware bandwidth. The number receive beams and their spacingdetermines the synthesized F-number on transmit. In one example, thehardware samples at 56 MHz with a 3.5 MHz per beam sampling rate andachieves F 3 transmit synthesis with 15 receive beams. Othercombinations are possible, such as a higher system sampling rate andlower per beam sampling to lower the synthesis F-number.

The receive beams are spaced according to 1-way Nyquist for theavailable receive aperture (λ/Aperture, e.g. 1.54 mm/us/(3.1 MHz*19mm)=1.5 degrees. The transmit beams are advanced according to 1-wayNyquist spacing for the to-be synthesized available transmit aperture(e.g. same as above). The frame rate is set due to transmit spacingdetermination (e.g. for 1.5 degree translation, providing about 80 Hz at160 mm). Assuming equal transmit and receive available aperture, thesynthesis configuration has a slip of one.

Analytic beam interpolation is used to provide an even greater number ofreceive beams. The sampling is at the receive Nyquist, and analytic beaminterpolation provides for well sampled round trip signal synthesis. Thesynthesis count (i.e., number of combined samples per location) is at aminimum of two (i.e., two or more at the deepest depth in the field ofview). The minimum count may be one (i.e., confocal imaging), but thelateral sampling requirement becomes 2-way, yielding a frame ratepenalty. A round-trip focus is provided for each location, and a higherframe rate than that resolution would imply is provided becauseapproximately half the lateral resolution is created in the imageformer.

With a minimum synthesis count of two, the transmit aperture issynthesized as Fnum=maxDepth/Aperture. In the near field, data steeredout to the angle indicated by the minimum synthesized F-number is used.These angles are provided by large (e.g., eight or more) receive beamcounts. The minimum synthesized transmit Fnumber (txFmin) for a givennumber of receive beamformer beams available (Nrxb) and receive beamspacing s is given by the formula txFmin=½ cotan(½ Nrxb*s), or for smallangles approximately 1/(s*Nrxb). (e.g. txFmin=2.35, or approximately2.39 for Nrxb=16, s=1.5 deg (0.026 rad from A=19, λ=1.54/3.1)). By thegeometry of the “look angle” for the progressing transmit focus, thetransmit focus position that provides synthesis of the full extent ofthe transmit aperture (A) for a given synthesis count (N) at the maximumdisplay depth (D) for 1-way Nyquist beam spacing s (radians) is given bythe formula TxDepth=A*D/{A−D*(N−1)s} (e.g. TxDepth=205 mm for A=19 mm,D=160 mm, N=2, s=1.5 deg(0.026 rad)). The formula is approximate, asD*(N−1)s is an arc length treated as a line segment for similartriangles, and it does not consider obliquity. The virtual point sourcefalls out of this transmit focus depth equation when D*(N−1)s>A.

The bandwidth of lateral frequencies that is contributed by eachacquisition is related to how close the curvature of the desired orsynthesized wavefront (equal to receive focus wavefront) is to thetransmit wavefront for that acquisition event. A rule, such as about ¼cycle or stationary phase integral approach, is used to model thecontribution of each transmit to the synthesized aperture. For deeperdepths, the synthesis count goes down, but the lateral bandwidthcontributed by each acquisition goes up. The transmit beams may bechosen so that, at the maximum depth, the synthesis count of twoprovides about the available lateral bandwidth of transmit separationbetween the two contributors.

For an even higher frame rate, the transmit steering may be made toprogress as the field of view is scanned by more than a would beindicated by the one way Nyquist criterion based on transmit aperture.Greater transmit beam coverage is then required, and the quality of theeffective synthesized transmit apodization may be compromised. Very highframe rates can be achieved while preserving high resolution, albeit atthe expense of other image quality metrics.

The above instructions are one example. Other examples with differentfrequencies, counts, or other retrospective transmit focus parametersmay be used.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

1. A method for retrospective dynamic transmit beamformation in medicalultrasound imaging, the method comprising: transmitting first and secondtransmit beams having static first and second foci, respectively, thefirst focus different than the second focus; generating first and secondsets of multiple receive beams in response to the first and secondtransmit beams, respectively, each of the receive beams having coherentsamples representing at least overlapping locations; for each of aplurality of overlapping locations, applying an at least one cycle delayoffset to at least one of the coherent samples, the offset being afunction of the first focus and a receive focus for the coherent sample;combining coherent samples from the first and second sets for eachlocation after applying the offsets, the coherent samples representingeach location being a function of the offset; and generating an imagethe image being a function of the combined coherent samples.
 2. Themethod of claim 1 wherein transmitting comprises transmitting the firstand second transmit beams with different transmit delay profiles, andwherein combining comprises combining for each location, the differenttransmit delay profiles having different regions of an aperture havingslopes similar to a receive delay slope for the location.
 3. The methodof claim 1 wherein the first and second foci have different lateralpositions.
 4. The method of claim 1 wherein the offsets correspond totemporal shifts in one of the transmit delay profiles to a tangentialintersection with receive wavefronts for the data.
 5. The method ofclaim 1 wherein a lateral spectrum of the combined coherent samples islinear phase over a greater range of frequencies than for the coherentsamples before combining.
 6. The method of claim 1 wherein a number ofthe sets contributing data to be combined varies as a function of depth.7. The method of claim 1 wherein a field of view of the image is betweena transducer used for the transmitting and the first and second foci,the first and second foci being outside the field of view and withinfour times a depth of the field of view.
 8. The method of claim 1further comprising phase aligning the coherent samples for each locationprior to the combining.
 9. The method of claim 1 further comprising:detecting information from the combined coherent samples; and low passfiltering and decimating the information; wherein the image is afunction of the filtered and decimated information.
 10. In a computerreadable storage medium having stored therein data representinginstructions executable by a programmed processor for retrospectivedynamic transmit beamformation in medical ultrasound imaging, thestorage medium comprising instructions for: acquiring data from multiplepulse-echo acquisition events, each pulse-echo acquisition eventcorresponding to different transmit delay profiles; aligning the datawith offsets, the offsets corresponding to a temporal shift in one ofthe transmit delay profiles to a tangential intersection with receivewavefronts for the data; combining the aligned data; and generating animage as a function of the combined data.
 11. The computer readablestorage medium of claim 10 wherein the offsets comprise element channeldelays, at least some of the channel delays being greater than a singlecycle.
 12. The computer readable storage medium of claim 10 wherein theoffsets comprise delays for beamformed data, at least some of the delaysbeing greater than a single cycle.
 13. The computer readable storagemedium of claim 10 wherein the offsets comprise phase adjustments. 14.The computer readable storage medium of claim 10 wherein combiningcomprises combining coherent data prior to detection.
 15. The computerreadable storage medium of claim 10 wherein the data representsdifferent locations in a field of view, and wherein combining comprisescombining for each location, the different transmit delay profileshaving different regions of an aperture having slopes similar to areceive delay slope for the location.
 16. The computer readable storagemedium of claim 10 wherein the different transmit delay profilescorrespond to foci having different lateral positions, and wherein theoffsets correspond to temporal shifts in one of the transmit delayprofiles to a tangential intersection with receive wavefronts for thedata.
 17. The computer readable storage medium of claim 10 wherein eachpulse-echo acquisition event corresponds to a set of receive data, thecombining being a combining of data from different sets, a number of thesets contributing data to be combined varying as a function of depth.18. A method for retrospective dynamic transmit beamformation in medicalultrasound imaging, the method comprising: transmitting a plurality oftransmit beams having different foci; generating sets of multiplereceive beams in response to the respective transmit beams, each of thereceive beams of a set having coherent samples representing differentlocations in a field of view, at least one receive sample of eachreceive beam set is common among the sets; for each location, applyingan offset to at least one of the coherent samples; combining coherentsamples from the sets for each location after applying the offsets, anumber of sets contributing data to be combined varying as a function ofdepth; and generating an image representing the field of view, the imagebeing a function of the combined coherent samples.
 19. The method ofclaim 18 wherein transmitting comprises transmitting the plurality withthe different foci in different lateral positions, and wherein thenumber is greater for a near field than a far field of the field ofview.
 20. The method of claim 18 wherein transmitting comprisestransmitting the transmit beams with different transmit delay profiles,and wherein applying comprises applying for each location, the differenttransmit delay profiles having different regions of an aperture havingslopes similar to a receive delay slope for the location as a functionof the offsets.
 21. In a computer readable storage medium having storedtherein data representing instructions executable by a programmedprocessor for retrospective transmit beamformation in medical ultrasoundimaging, the storage medium comprising instructions for: transmitting aplurality of transmit beams having different foci; generating sets ofmultiple receive beams in response to the respective transmit beams,each of the receive beams having coherent samples representing locationsin at least a same region; for each location, retrospectively focusingthe transmit beams to the location by altering the coherent samples;combining coherent samples from the sets for each location afterretrospectively focusing; and generating an image, the image being afunction of the combined coherent samples.
 22. An ultrasound imageformation method comprising: forming plurality of receive beam sets,each set using echoes received in response to a transmit event withspatially distinct phase front and each set representing at least acommon region as another set; aligning, in depth, receive data for aplurality of the sets prior to amplitude detection, the aligningproviding, for each location, receive delay slopes for different regionsof an aperture having slopes similar to a receive delay slope for thereceive beams; and generating a synthesized line through pre-detectionsummation of depth aligned receive data.
 23. The method of claim 1further comprising weighting the coherent samples prior to thecombining, weights being a function of a transmit apodization profile.24. The computer readable media of claim 10, wherein the instructionscomprise weighting the data prior to the combining, weights being afunction of a transmit apodization profile.